Small size insulated wire is commonly called magnet wire since it used most often in winding for electromagnets and generators. It is also used to make electronic components such as inductors and transformers. In addition, it is used for lead wires in instrumentation and related applications. To be used in these applications, there are a number of properties the wire must possess: it must be small, pliable for routing and winding, and well insulated from electrical contact. In addition, it is advantageous if the wire can survive high temperatures as this often allows more efficient and economical designs. In some cases, a degree of chemical resistance is also required. Where used in weight critical components, such as aircraft generator coils, a thin, electrically insulated wire with high thermal tolerance is required.
Insulating coatings commonly used for magnet wire include extruded PTFE (Polytetrafluoro Ethylene) or Teflon and FEP (Fluorinated Ethylene Propylene) insulation. These insulations only add about 0.0007″ to the wire outer diameter (OD), and leave the wire readily pliable for routing or winding. The highest service temperatures for these coatings are 392° F. for FEP and 428° F. for PTFE.
Higher service temperatures (up to 840° F.) can be reached by non-extruded insulating materials, most commonly a wrap of mica tape in fiberglass sheath. Since this combination is permeable to gas, copper conductors are prone to severe corrosion. To avoid this problem, providers of the wire product use strands of nickel clad copper wire. This results in the smallest available wire being 22 AWG with a nominal 0.0253″ OD. The addition of the mica wrap and fiberglass braid makes the 22 AWG insulated product diameter grow to 0.071″. The smallest mica-fiberglass wire available is 18 AWG, which has a total OD 0.097″, and is less pliable and more difficult to route than thinner wires.
Still higher service temperature can be reached by using ceramic insulated wire or manganese oxide insulation inside a metal sheath. However, these insulations have problems as well. The ceramic insulation that is commonly available is porous, so it absorbs any liquids or gasses it contacts. It is also relatively easy to remove from the wire, so caution must be exercised when handling or spooling it. The metal-manganese oxide insulation can withstand extremely high temperature (up to 2000° F. in some cases), however, the use of a comparatively stiff metal sheath makes it extremely difficult to route or spool. It also has a large OD compared to the size of wire it insulates.
It is therefore desirable to find an insulation that can withstand comparatively high temperatures, adds little to the diameter of the wire, is relatively tough, and is nonporous and pliable. Parylene HT is one such insulation material. Parylene HT does not melt, but starts to oxidize at temperatures greater than 450° C. (842° F.) and becomes brittle.
Parylene HT is the trade name for one of a family of polyxylylene polymers of which various members have been sold under the name Parylene since the 1970s. Parylenes are particularly valued for their high dielectric strength, high chemical resistance, low permeability and ability to thoroughly coat small surfaces and enter small spaces. Parylene HT forms very thin and tough coatings and can withstand high temperature, making Parylene HT uniquely suited for insulating magnet wire.
However, Parylene HT requires an exotic application process, wherein the object to be coated is placed in a vacuum chamber and Parylene HT is introduced as a vapor for 6 to 8 hours. Parylene HT cannot therefore be applied to magnet wire using the conventional high volume reel-to-reel wire coating process.
For further information on Parylene HT, see U.S. Pat. No. 7,462,750 of Specialty Coating Systems of Indianapolis, Ind.